Vehicles normally have vehicle headlight devices for illuminating areas in front of the vehicles. ‘Front’ means in a travel direction of a vehicle. To avoid unnecessary illumination of oncoming vehicles and preceding vehicles, an area illuminated by a vehicle headlight device is generally in a range of a vertically narrow and horizontally wide light distribution pattern. When a vehicle tilts as it turns a corner, however, the vehicle headlight device tilts together with the vehicle body. Therefore, there has been a problem of inadequate illumination in a corner area toward which the driver's gaze is directed.
For example, when a motorcycle makes a left turn at a corner, the motorcycle body tilts to the left. When this happens, a light distribution of a headlight device (headlamp) tilts down to the left and up to the right. The ‘light distribution’ indicates what intensity of light is emitted in what direction (angle) from a light source. The light distribution is accordingly a luminous intensity distribution of a light source with respect to a space. In other words, the light distribution is a spatial distribution of light emitted from a light source. At this time, a position lower than a corner area toward which a driver's (rider's) gaze is directed is illuminated. The corner area that should be illuminated therefore becomes dark. The corner area is an area in a travel direction of the vehicle. On the contrary, on the right side of the road in the travel direction, a position higher than the road surface is illuminated. So, it is worried that glaring light illuminates an oncoming vehicle. Incidentally, when the motorcycle body tilts to the left, because the motorcycle turns to the left, the corner area is on the left side of the road in the travel direction.
As a method of solving these problems, a method has been disclosed. The method includes a lamp body in which a light emitting body and a lens disposed in front of it are provided so as to be freely rotatable, a driving mechanism which rotates the light emitting body and the lens, a bank angle sensor which senses a bank angle of a vehicle body, and a rotation controller, and it is a method of rotating the light emitting body and the lens on the basis of the bank angle obtained from the bank angle sensor. (See, for example, patent reference 1). The ‘bank angle’ is an angle when a vehicle body is tilted from an upright position. The bank angle is measured as 0 degrees when the vehicle body stands upright and as 90 degrees when it is turned on its side.